Anterior cruciate ligament (ACL) tear injuries are among the most common injuries of the knee. In the US alone, there are over 200,000 ACL tear injuries every year. 
The ACL is a ligament that connects the upper and lower leg, and it is located within the knee joint. The blood supply is provided by arteries surrounding the knee joint and fat pad within the knee joint. Blood supply is very important during the recovery process after the injury. The main role of the ACL is to provide stability to the knee joint and prevent excessive movement of the lower leg anteriorly. 
Causes Of An ACL Tear Injury
In most cases of an ACL tear injury, the patient either suffered a hit to the knee or was involved in activities like sudden deceleration, cutting action, or jumping. Researchers have found that the width of intercondylar notch (the space between two condyles of the femur) and gender play a significant role in developing an ACL tear injury. 
ACL tear injuries can be divided into four main grades (intact, Grade I, Grade II, Grade III).
- In the case of Grade I ACL tears, only some of ligament fibers are torn while the rest are either intact or distended.
- In the case of Grade II ACL tear injuries, a significant amount of ligament fibers are torn, but there is still a link between the two ends of the ACL.
- In the case of Grade III ACL tear injuries, all ligament fibers are torn and there is no link between the two ends of the ACL. 
Symptoms Of ACL Tear Injuries
When providing information regarding the injury, most patients will describe a popping sensation which can be either heard or felt. It is accompanied by a sharp pain that disables them from continuing their activity. A lot of patients also describe a sensation “like their knee was dislocated and then went back to its place".
Shortly after the injury, blood will start collecting within the knee joint and the knee will start swelling. In case of larger amounts of blood within the joint, the patient might feel pain and tightness in the knee even while resting. The range of motion will be significantly reduced and even slight flexion will cause pain.
The swelling often creates a problem because it hinders a thorough examination. The examination includes different orthopedic tests which can guide the examiner towards the exact diagnosis. In case of ALC tear injury, the Lachman test, the pivot shift test, and anterior drawer test are the most sensitive. 
Surgery For ACL Tear Injuries
There are two main treatment options for an ACL tear injury: conservative treatment and surgery. The choice of treatment modality depends on a number of different factors (grade of ACL tear, age of the patient, level of activity prior to injury, willingness to change lifestyle, cost efficiency, etc.). The three main factors to decide on a surgical treatment are:
- The patient's age
- The patient's activity level prior to surgery
- The extent of instability of the knee joint. 
Patients with a Grade I ACL tear injuries and even patients with Grade II ACL tear injuries who are prepared to change their lifestyle might consider conservative treatment. In case this conservative treatment fails, they can have the surgery for ACL tear injury.
In the case of Grade III ACL tear injuries, there is no link between the two ends of the ACL and there is no chance for healing without surgery.
Depending on the type of ACL tear, level of activity and other factors, an orthopedic surgeon will choose between ACL repair and ACL reconstruction.
ACL repair is reserved for cases in which the ACL abrupted from its attachment to the femur or tibia. An orthopedic surgeon will place the avulsed part back to its normal position and fix it with a suture or screw. Bigger fragments can be fixed using a screw, while smaller ones are fixed with nonabsorbable sutures. There is no difference between the success rates of screws and sutures.
ACL reconstruction surgery is recommended for patients who suffered an ACL tear located in the middle part of the ligament. A graft is used to replace injured ACL. Most surgeons use patellar tendon, hamstrings tendons, quadriceps tendon or synthetic grafts. A study performed by Spindler et al. found no significant difference between patellar tendons and hamstrings tendons.
Surgery Recovery Time
Most orthopedic surgeons recommend using a brace and crutches after the ACL repair. Three weeks after the surgery, the patient will be allowed to start with rehabilitation exercises for ACL tear injury to strengthen the quadriceps muscle and hamstrings. They will be allowed to flex his knee up to 90 degrees using a brace. The general recommendation is to continue with exercises during first three to four months after the ACL repair.
The patient should be able to have a full range of motion two months after the surgery. Crutches are used during the first six weeks after the surgery.
In case of ACL reconstruction surgery, the knee is placed in a brace in full extension. The patient is allowed to start with exercises right after the surgery. Most orthopedic surgeons recommend using a six months recovery protocol.